In November 1941, not long before the attack on Pearl Harbor, a handful of letters began materializing in student mailboxes at America’s top women’s colleges. The messages were cryptic and brief, inviting their mystified recipients to private interviews in which the students might be asked only a couple of similarly cryptic questions: Did they like crossword puzzles, and were they engaged to be married? The correct answer to the first question was yes; the desired answer to the second was no.

For more than a year, the U.S. Navy had been quietly recruiting male intelligence officers—specifically, code breakers, or “cryptanalysts”—from elite colleges and universities, and now it was embarking on the same experiment with women. Educated women were wanted for the war effort, and with all possible haste.

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In early meetings, the chosen women were issued manila envelopes containing a brief introduction to the arcane history of codes and ciphers, along with numbered problem sets they were to complete every week. In the late spring of 1942, the first wave of women recruited by the Navy finished their secret courses and turned in their final problem sets. Those who had stuck with the course and answered enough problems correctly—less than half of those recruited—arrived in fresh cotton dresses, prepared to start their duties, working in the Navy’s hot and cramped downtown Washington, D.C., headquarters.

Around the same time, another meeting was taking place. Twenty women’s colleges sent representatives to the elegant Mayflower Hotel in Washington, D.C., where the U.S. Army’s own code-breaking operation was working to forge ties with institutions that schooled women, scrambling to recruit their top seniors before the Navy, or any other agency or service, did. The inspector general of the Department of Labor noted that adult civilians would not be sufficient to stock an economy bereft of its male workers, many of whom had shipped out to battle. And so, in the intense and chaotic atmosphere of wartime America, another, smaller war was taking place: A war in which female college students, for the first time in U.S. history, were not only recruited by employers, but competed for.

Disparate as their backgrounds were, the women who answered the Navy’s and Army’s code-breaking summonses had a handful of qualities in common. They were smart and resourceful, and they had strived to acquire as much schooling as circumstances would permit, at a time when women received little encouragement or reward for doing so. They were adept at math or science or foreign languages, often all three. They were dutiful and patriotic. They were adventurous and willing. And they did not expect any public credit for the clandestine intelligence work they were entering into.

This last fact was perhaps the most important: Their work was to be kept entirely secret. Just because they were female, they would not be spared the full consequences for treason in wartime. If they went out in public and were asked what they did, they were to say they emptied trash cans and sharpened pencils. One group agreed that if anybody ordered a vodka Collins when they were out at a bar together in Washington, that would be their signal that a stranger was showing too much curiosity about their work, and they were all to disperse to the ladies’ room and then flee. Some women would improvise answers, replying lightly that they sat on the laps of commanding officers. People readily believed them. For a young American woman, it was all too easy to convince an inquiring stranger that the work she did was menial, or that she existed as a plaything for the men she worked for.

“Almost everybody thought we were nothing but secretaries,” one of the women would say years later.

That millions of women served the war effort by rolling up their sleeves and donning trousers and jumpsuits to work in factories—the celebrated Rosie the Riveter, who helped build bombers and tanks and aircraft carriers—is well-known. Far less well-known is that more than 10,000 women traveled to Washington, D.C., to lend their minds and their hard-won educations to the war effort—and were behind some of the most significant individual code-breaking triumphs of the war. The women came from a generation when women did not expect—or receive—credit for achievement in public life. They did not constitute the top brass, and they did not write the histories afterward, nor the first-person memoirs. Their efforts were almost completely hidden for more than 70 years, their contributions mentioned only in passing. That their story is now being told is the result of three years’ of book research, including in the National Archives in College Park, Maryland, and many other archival repositories, as well as declassification requests and interviews with living code breakers—some of whom are reluctant, even now, to use certain words they had been told never to utter outside the code-breaking compounds.

As it turns out, during the most violent global conflict that humanity has ever known—a war that cost more money, damaged more property, and took more lives than any war before or since—these women formed the backbone of one of the most successful intelligence efforts in history, an effort that began before the Pearl Harbor attack and lasted until the end of World War II. And after the war, the Army and Navy code-breaking operations merged to become what is now the National Security Agency. It was women who helped found the field of clandestine eavesdropping—much bigger and more controversial now than it was then—and it was women in many cases who shaped the early culture of the NSA.

Not that their success was guaranteed. In the manila packets they opened before arriving in Washington, the women were told that, up to now, the secret cryptanalytic work they had been selected to perform had been done by men.

“Whether women can take it over successfully,” the Navy letter told them, “remains to be proved.” The letter added: “We believe you can do it.”

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In Washington, female code breakers were instrumental at every stage of the war. They ran complex office machines that had been converted to code-breaking purposes. They built libraries of public speeches, shipping inventories and lists of ship names and enemy commanders, which helped break messages and illuminate their content. They worked as translators. Women were put in charge of “minor” systems—weather codes, for instance—that turned out to be crucial when major systems went dark and could not be read.

And a number of predominately female teams attacked and broke major code systems. Once broken, a code must be exploited and, often, rebroken, and women formed the great assembly line of workers who did this. Women also tested America’s own codes to make sure they were secure. They worked as radio intercept operators at global listening posts. The Navy did not permit its women to serve overseas—much as many wanted to—apart from a few who went to Hawaii, but the Army did admit its code-breaking women into the war theater. Some Army women would be sent to Australia and to Pacific islands such as New Guinea. Some would move with General Douglas MacArthur when he occupied Tokyo after the war. Other women helped create “dummy traffic”: fake radio signals that helped fool the Germans into believing the D-Day invasion would take place in Norway or the Pas-de-Calais region of France—rather than on the beaches of Normandy.

These were the formative days of what is now called “information security,” when countries were scrambling to develop secure communications at a time when technology was offering new ways to encipher and conceal. As in other nascent fields, like aeronautics, women were able to break in largely because the field of code-breaking barely existed. There had not yet been put in place elaborate systems of regulating and credentialing—professional associations, graduate degrees, licenses, clubs, learned societies, accreditation—the kinds of barriers long used in other fields, like law and medicine, to keep women out.

Women also were put in a peculiar position by dint of being brought into the workforce to free up men for military service. “Release a man to fight” was the phrase of the day. As a result, men who had been doing office work sitting at desks were able to ship out to the violent war theater.

Women were considered better suited for code-breaking work by many people—but alas, this wasn’t a compliment. What this meant was that women were considered better equipped for boring work that required close attention to detail rather than leaps of genius. In the field of astronomy, women long had been employed as “computers,” assigned to do lower-level calculations. This was seen as women’s rightful domain: The careful repetitive work that got things started, so that the men could take over when things got interesting and hard. Men were seen as more brilliant than women, but more impatient and erratic. “It was generally believed that women were good at doing tedious work—and as I had discovered early on, the initial stages of cryptanalysis were very tedious, indeed,” recalled Ann Caracristi, whose first job as a code breaker was sorting reams of intercepted traffic. But all along there have been female geniuses whose contributions are as important as those of men. It’s just that far less attention has been paid to them, and often these women were denied the top spots that would have brought them more recognition.

At Arlington Hall in Virginia, Ann Caracristi (far right), an English major from Russell Sage College, matched wits against Japanese code makers, enabling military intelligence to develop “orders of battle” showing the location of Japanese troops. | National Security Agency

Women had a strike against them in that they were considered bad at keeping secrets—women, everybody believed, were gossips, rumormongers, talkers. Then again, when it came to sexual behavior, women were seen as less of a security risk than men. Just before the United States entered the war, when the Army began recruiting privates to train as radio intercept operators, an internal memo raised a concern about the ramifications of entrusting young men to do top secret work. Youth, the memo noted, is “a time for sowing of wild oats and under the influence of women and liquor, much is said that the speaker would not dream of saying when uninfluenced.” Women were thought to be less problematic, at least when it came to drinking and bragging.

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How significant were these women’s efforts?

The Axis powers never mobilized their women to the extent that the Allies did. Japan and Germany were highly traditional cultures—the Nazis saw women as breeders—and women were not pressed into wartime service in the same way, not for code-breaking or other high-level purposes. There are of course many reasons the Allies prevailed in World War II—the industrial might of the United States, the leadership of military commanders and statesmen, the stoicism of British citizens who endured years of bombing and deprivation, the resistance of the French and Norwegian undergrounds, the cunning and resourcefulness of spies, the heroism of citizens who helped and harbored Jewish neighbors, and the bravery and sacrifice of sailors and airmen and soldiers, including the millions of Soviet soldiers who bore the brunt of military casualties and deaths.

But the employment of women also was one of these factors. It wasn’t just that the women freed the men to fight, enabling General Dwight Eisenhower to load more men into landing craft at Normandy, or Admiral Chester Nimitz to staff more Pacific aircraft carriers. Women were active war agents. Through their brainwork, the women had an impact on the fighting that went on.

Also at Arlington Hall, a secret African-American unit—mostly female, and unknown to many white workers—tackled commercial codes, keeping tabs on which companies were doing business with Adolf Hitler or Mitsubishi. | National Security Agency

Listening in on enemy conversations provided a verbatim, real-time way to know what that enemy was thinking and doing and arguing about and worrying over and planning. It provided information on strategy, troop movements, shipping itineraries, political alliances, battlefield casualties, pending attacks and supply needs. The code breakers of World War II advanced what is known as signals intelligence—reading the coded transmissions of enemies, as well as (sometimes) of allies. They laid the groundwork for the now burgeoning field of cybersecurity, which entails protecting one’s data, networks, and communications against enemy attack. They pioneered work that would lead to the modern computing industry.

The women also played a central role in shortening the war. Code-breaking was crucial to Allied success in defeating Japan, a foe that was willing to fight to the death. And in the all-important Atlantic theater, U.S. and British penetration of the Nazi Enigma cipher that German Admiral Karl Donitz used to direct his U-boat commanders helped bring about the total elimination of the Nazi submarine threat.

On the eve of Pearl Harbor, the Army had 181 people working in its small, highly secret code-breaking office in downtown Washington. By 1945, of the Army’s 10,500-person-strong code-breaking force—relocated into much larger quarters—nearly 70 percent was female. Similarly, at the war’s outset the Navy had a few hundred code breakers. By 1945, there were 5,000 stationed in Washington, and about the same number serving overseas. At least 80 percent of the Navy’s domestic code breakers—some 4,000—were female. Thus, out of about 20,000 total American code breakers during the war, some 11,000 were women.

Many of the program’s major successes finally did become public at the end of the war. Late in 1945, the New York Times published a letter that General George Marshall had written to Thomas Dewey, laying out some of the victories the country owed its cryptanalytic forces and begging Dewey to keep them secret. Once the war was over, the letter was made public. In it, Marshall pointed out that thanks to the country’s cryptanalytic forces, “we possessed a wealth of information” regarding Japanese strategy. He revealed the hidden cause of certain famous naval victories and pointed out that “operations in the Pacific are largely guided by the information we obtain of Japanese deployments.”

This article is excerpted from Code Girls: The Untold Story of the American Women Code Breakers of World War II.

Also after the war, the Joint Committee on the Investigation of the Pearl Harbor Attack noted that Army/Navy signals intelligence was “some of the finest intelligence available in our history” and that it “contributed enormously to the defeat of the enemy, greatly shortening the war, and saving many thousands of lives.” Major General Stephen Chamberlin announced that military intelligence, most of which came from code-breaking, “saved us many thousands of lives” in the Pacific theater alone, “and shortened the war by no less than two years.”

Members of Congress were quick to commend the code-breaking forces. “Their work saved thousands of precious lives,” orated Representative Clarence Hancock of New York, speaking on the floor of the House on October 25, 1945. “They are entitled to glory and national gratitude which they will never receive. We broke down the Japanese code almost at the beginning of the war, and we knew it at the finish of the war. Because of that knowledge we were able to intercept and destroy practically every supply ship and convoy that tried to reach the Philippines or any Pacific Island.”

Hancock stated, “I believe that our cryptographers ... in the war with Japan did as much to bring that war to a successful and early conclusion as any other group of men.”

That more than half of these “cryptographers” were women was nowhere mentioned.